We related normalized diurnal weight gain (NDWG) to a decrease in serum sodium concentration (DSOD) among 8 schizophrenics subject to water intoxication. DSOD = 0.422 + 1.675 × NDWG (n = 8, r = 0.864, P = 0.006). We used this relationship to interdict drinking and, thus, prevent severe hyponatremia.